Introduction: Gastroesophageal reflux (GER) is the involuntary passage of stomach contents to the esophagus with or without regurgitation and vomiting. Studies conducted in Indonesia showed that episodes of regurgitation persisted longer and its incidence was 2 times higher than other ethnics even in 12 months old. In fact, prolonged regurgitation may lead to gastroesophageal reflux disease. This study aims to evaluate exclusive breastfeeding as protective factor toward incidence of GER in infants aged 0-1 month old. Materials and method: This study was a prospective cohort study involving all infants aged 0-1 month old in several hospitals at Surabaya, Sidoarjo, and Gresik. Subjects were classified into 2 groups: exclusively breastfed groups and non-exclusively breastfed groups. Diagnosis of GER was made from history taking and physical examination, according to Rome IV criteria. Follow-ups were conducted at 3, 6, and 12 months in order to know if subjects showed any symptoms of GER or not. Results: A total of 222 subjects followed this study until 12 months. Result showed that fewer infants under exclusively breastfed group showed symptoms of GER after 6 months duration or more. Incidence of GER in both exclusively breastfed group and non-exclusively breastfed group did not differ significantly at 3 months. Conclusion: Exclusive breastfeeding exhibited protective effect toward GER in infants aged 0-1 year. Meanwhile, non-exclusively breastfed infants either bottle-fed with breastmilk, formula-fed, or any combination between breastfed and formula-fed, increased risk for GER by 1.45 times. Therefore, it is highly recommended to give exclusive breastfeeding for infants.